Insects pests and pathogens compromise the persistence of two endemic and rare Braya (Brassicaceae)

Rare and threatened plant species face a variety of threats to their persistence including habitat degradation, non-native herbivores, and pathogens. In this study we explored the effects of a non-native, agricultural pest and three pathogens on two rare vascular plants restricted to a unique ecosystem. Agro-ecosystems support many non-native insects, but their potential to find and impact rare, native plants is largely unknown. Plutella xylostella L. (diamondback moth) is a global agricultural pest of the Brassicaceae family, including the endangered Braya longii (Fernald) (Long's braya) and threatened B. fernaldii (Abbe) (Fernald's braya) that are endemic to the limestone barrens of Newfoundland, Canada. The immigration of P. xylostella from southern overwintering sites to this unique ecosystem was monitored with pheromone traps between 2003 and 2005. At the same time individually tagged Braya were monitored for the presence and impact of P. xylostella and three pathogens. Since habitat loss and deterioration is still the most important threat to the persistence of endangered species, the frequency of each pest was compared between Braya populations growing on anthropogenically disturbed and undisturbed habitat. -- Between 2003 and 2005, 30% of B. longii and 16% of B. fernaldii were infested by P. xylostella, 8.6% of the total B. longii population died from root rot (Fusarium sp.), 18% of B. longii on anthropogenically disturbed sites were infected with an unidentified pathogen causing their flowering stalks to rot, and 27% of B. fernaldii in northern sites were infected with an unidentified pathogen causing flowering stalk and leaf deformities. Impacted plants contributed between 9% and 75% less seeds to annual seed production than healthy, flowering plants and had a statistically higher probability of mortality. The majority (66%-100%) of pathogen infections occurred on anthropogenically disturbed habitat. -- Stage based transition matrices created from these data and summarized into deterministic projections predict Braya populations will decline over the next 10 years. P. xylostella may negatively impact the persistence of other rare Brassicaceae worldwide because they can infest rare plants growing in native vegetation, especially when the vegetation is sparse, and they, as do the pathogens, preferentially damage flowering plants. Modelling suggests that the management of pathogens in anthropogenically disturbed populations will most improve the population growth rate, where as the management of P. xylostella in undisturbed populations will most improve the population growth rate. Presently, insufficient attention is directed to the impacts of both native and non-native agricultural pests on rare host plants; hence, there is a need for both the conservation and agricultural communities to cooperate in mitigating their impacts on native biodiversity